Carbon and graphite fibers are desirable for use in metal-matrix composites because of their high-temperature strength, high elastic modulus, and low density. Yet their use in certain matrices, such as those containing nickel, is limited by the degradation of their mechanical properties.
In the fiber interface of a nickel-containing matrix, for example, exposure of the composite to temperatures in excess of 600.degree. C. for extended periods of time significantly reduces the tensile strength of the graphite fibers contained therein. The two mechanisms primarily responsible for the reduction of mechanical properties are reported to be: (1) decrease of fiber diameter due to diffusion of carbon from the fiber into the nickel matrix (as reported by Barclay R. B., J. Mater. Sci. 1971, 6, 1076-1083 and (2) nickel-catalyzed regraphitization of the fiber (as reported by Jackson, P. W. and Marjoram, J. R., Nature 1968, 218, 83-84). If the material is thermally cycled, the mismatch of thermal coefficients of expansion of the fiber and matrix material is also responsible for reduction in tensile strength. (as reported in U.S. Pat. No. 3,796,587)
Under constant thermal conditions, however, a diffusion barrier to suppress the interdiffusion of nickel and carbon has been reported to protect the mechanical properties of nickel-graphite matrices. Several such barriers have been proven to be beneficial, most notably those containing carbide-forming metals. Carbide-containing barrier coatings have been produced by a variety of methods, such as: by chemical vapor deposition (CVD) (Aggour, L., Carbon 1974, 12, 358-362); by soaking the fibers in a melt containing a carbide forming metal and an acid soluble metal, followed by an acid bath to remove the unreacted metal (U.S. Pat. No. 3,796,587); by precoating the fiber in a melt containing a refractory powder suspended in a low-melting-point metal [Rashid, 1974]; and by electrodeposition of a thin layer of nickel followed by another thinner layer of a carbide-forming metal (U.S. Pat. No. 3,807,996). The last two methods required the subsequent heating of the coated fibers to allow diffusion of the reactive metal to the graphite surface where a carbide interfacial zone was formed and therefore involves undesirable processing steps. Yagubets and Sherstkina (Elektron, Obrab, Mater, 1974, 5, 31-33, CA 82:117765n) electrodeposited, plastic cobalt-tungsten and nickel-tungsten coatings on graphite fibers from an aqueous bath.